On the inner surface of a pipe, the outer surface of which is exposed, installed on the ground for transporting, for example, a fluid such as a gas or a liquid, a defective portion such as those listed below may be present:
(1) an accumulation of foreign matters resulting from deposition of foreign matters such as rust and scale, and
(2) a thinner portion caused by corrosion and the like.
Presence of an accumulation of foreign matters as mentioned above on the inner surface of a pipe causes such problems as impediment of smooth flow of the fluid in the pipe. Presence of a thinner portion as mentioned above on the inner surface of the pipe ultimately results, on the other hand, in such problems as production of a hole at the thinner portion. It is therefore necessary to promptly detect the above-mentioned defective portion on the inner surface of the pipe, and replace the pipe having the defective portion with a new one.
The following methods are known for detecting a defective portion on the inner surface of a pipe, the outer surface of which is exposed:
(1) Detecting method using radioactive rays:
This method comprises projecting radioactive rays such as X-ray or gamma-ray toward a pipe to be tested from the side of the outer surface thereof, measuring the amount of radioactive rays having passed through the pipe, and detecting a defective portion on the inner surface of the pipe by means of the amount of transmission of the radioactive rays.
(2) Detecting method using ultrasonic waves:
This method comprises transmitting ultrasonic waves toward a pipe to be tested from the side of the outer surface thereof, receiving reflected waves of the transmitted ultrasonic waves, measuring the time required up to receiving of the reflected waves of the transmitted ultrasonic waves, and detecting a defective portion on the inner surface of the pipe by means of the time required up to receiving of the reflected waves.
(3) Detecting method based on knocking:
This method comprises knocking a pipe to be tested from the side of the outer surface thereof with a hammer, for example, and detecting a defective portion on the inner surface of the pipe by means of the thus produced sound.
(b 4) Detecting method based on cutting:
This method comprises cutting out a pipe to be tested to permit an operator to directly observe the inner surface of the pipe, thereby detecting a defective portion on the inner surface of the pipe.
The detecting method using radioactive rays as mentioned in (1) above has the following problems
(a) Detecting operation of a defective portion cannot be conducted unless the operator is qualified for handling radioactive rays. There is therefore a limitation in personnel.
(b) It is difficult to make a proper judgement on the result of detection, requiring high-level experience and technical knowledge.
(c) Detecting operation of a defective portion can be carried out only at a position closest to the outer surface of the pipe. When the pipe is installed at an elevated position apart from the ground, therefore, it is necessary to provide a scaffold for detecting operation.
(d) The range of a single run of detection is narrow. It thus requires much time and labor for detecting operation, leading to a low operating efficiency.
The detecting method using ultrasonic waves as mentioned in (2) above has the following problems
(a) An error is often contained in the result of detection of a defective portion, thus preventing accurate detection.
(b) Detecting operation of a defective portion can be carried out only at a position in contact with the outer surface of the pipe. It is therefore difficult to conduct detecting operation when a high-temperature fluid flows through the pipe. When the pipe is installed at an elevated position apart from the ground, furthermore, it is necessary to provide a scaffold for detecting operation.
(c) The range of a single run of detection, being only a point, is very narrow. It thus requires much time and labor for detecting operation, leading to a low operating efficiency.
The detecting method based on knocking as mentioned in (3) above has the following problems
(a) Determination of the presence of a defective portion requires high-level experience and technical knowledge, with furthermore much differences between individual operators, thus impairing accurate detection of the defective portion.
(b) Detecting operation of a defective portion can be carried out only at a position closest to the outer surface of the pipe. When the pipe is installed at an elevated position apart from the ground, therefore, it is necessary to provide a scaffold for detecting operation
(c) Detecting operation requires much time and labor, resulting in a low operating efficiency.
(d) It is difficult to detect a small defective portion.
The detecting method based on cutting as mentioned in (b 4) above has the following problems
(a) Use of the pipe must be interrupted for a while during cutting and detecting operations of a defective portion, and these cutting, detecting and restoring operations require much time and labor, resulting in a low operating efficiency.
(b) When the pipe is installed at an elevated position apart from the ground, it is necessary to provide a scaffold for detecting operation.
Under such circumstances, there is a strong demand for the development of a method and an apparatus which permit certain, easy and efficient detection of an accumulation of foreign matters or a thinner portion as a defective portion on the inner surface of a pipe, the outer surface of which is exposed, at a position apart from the pipe in a non-contact manner without the need for a special qualification, but a method and an apparatus provided with such properties have not as yet been proposed.